All of my input on this is purely anecdotal, based on my own personal experiences and the experiences of others that I’ve come across. At no point do I claim to be an expert on flatworms, or anything for that matter.
I think you would need a serious infestation of flatworms, do absolutely zero removal of the dying ones, and run little to no carbon for it to cause a major adverse reaction.
I’ve used FWE a total of three times. The third time, my tank was absolutely infested. You couldn’t find a surface in the tank that didn’t have flatworms on it. I dosed something like 2-3 times the recommended dosage, ran lots of carbon, and siphoned them out as the FWE worked its magic. I cant comment on the effect it has on fish because I didn’t have any at the time. The corals didn’t react even a little, save for my Duncan that was back to normal in 5-10 minutes.
Even then, I personally wonder if it’s the toxicity from the flatworms that really causes an issue. Wouldn’t that be an issue for the wrasses that are so commonly employed to eradicate them? Or is it just that the toxin is very mild and only really a danger in large quantities?
Do fish somehow build up an immunity? After round 3, the flatworms quickly returned to plague proportions and were left alone for a few months. They didn’t bother anything, they were just ugly: I finally introduced a leopard wrasse and haven’t seen a single flatworm since the first week he was in the tank. It’s been ten months.
That fish ate every flatworm it could find and seems no worse for the wear.
